Filter that generates an ETag value based on the content on the response.
This ETag is compared to the If-None-Match header of the request. If these headers are equal,
the response content is not sent, but rather a 304 "Not Modified" status instead.

Since the ETag is based on the response content, the response (or View)
is still rendered. As such, this filter only saves bandwidth, not server performance.

Same contract as for doFilter, but guaranteed to be
just invoked once per request. Provides HttpServletRequest and
HttpServletResponse arguments instead of the default ServletRequest
and ServletResponse ones.